Recently, there has been a great demand for polyacetals as engineering plastics because of their superior mechanical characteristics, fatigue characteristics and frictional wear characteristics. However, with diversification of uses and demand for enhancement of functions of polyacetals, properties possessed by polyacetals cannot satisfy all of the requirements.
In order to improve this point, extensive research has been conducted on modification of polyacetals, for example, a polymer alloying technique where a polyacetal and a resin other than a polyacetal are blended or a technique to modify the skeleton by introducing a component other than a polyacetal into the main chain of a polyacetal.
For example, as an example of the polymer alloying technique, EP-A-115373 discloses that a composition obtained by adding to a polyacetal an acrylic multi-phase interpolymer having a two-phase structure and having a particle size of 10-100 .mu.m is excellent in impact resistance.
However, the acrylic multi-phase interpolymer disclosed in the above patent publication has a rigid thermoplastic phase as the outermost phase and since affinity of this outermost phase to a polyacetal is insufficient, the composition cannot be said to be sufficient in high impact resistance required for engineering plastics. Further, the composition has the defect that impact resistance in a specific direction decreases significantly depending on processing conditions.
Specifically, impact resistance in a specific direction decreases when processing is conducted under conditions which apply orientation to the multi-phase interpolymer dispersed in the polyacetal in processing such as injection molding, extrusion molding, and blow molding. For example, in a weld portion which is formed by joining molten resins to each other in a mold for injection molded products, impact resistance in a specific direction of the molded product, namely, weld strength greatly decreases. This phenomenon occurs due to the fact that the multi-phase interpolymer dispersed in the polyacetal is not uniformly dispersed in all portions of the molded product, but non-uniform un-uniform dispersion such as agglomeration or orientation occurs in the molded product.
Therefore, impact resistance of the composition disclosed in the above patent publication is non-uniform and compositions having mechanical properties excellent in isotropy cannot be obtained.
The mechanical properties excellent in isotropy in the present invention mean mechanical properties which show no reduction in tensile characteristics and impact resistance in a specific direction in any portion, especially a weld portion, of molded products and having uniformity.
Examples of the technique to modify the skeleton for solving the above problems are as shown below.
For example, U.S. Pat. No. 4,535,127 discloses that graft copolymers having a structure where a polyacetal is grafted on elastomers such as modified ethylene-propylene copolymer and modified styrene-butadiene copolymer have high impact resistance.
Moreover, U.S. Pat. No. 4,535,127 discloses that A-B-A triblock copolymers comprising polyacetal units (A) and elastomer units (B) have excellent impact resistance.
However, in these processes, normally, copolymerization is carried out in a system where the elastomer is completely dissolved in polymerization solvents or the like. Therefore, the resulting acetal copolymer just after being copolymerized is in a viscous slurry state and there is the problem that filtration and drying of the acetal copolymer are difficult and when the processes are industrially carried out, it is very difficult in the manufacturing technique to stably mass-produce acetal copolymers. Besides, since this problem becomes conspicuous with an increase in the amount of elastomer introduced into the acetal copolymer, acetal copolymers having a large introduction amount of elastomer cannot be produced and this is a serious obstacle in improving the impact resistance of acetal copolymers.
Furthermore, according to the processes of the above patent, acetal copolymers just after copolymerization are a viscous slurry state and hence, in a short time, the polymers stick to the wall of the polymerization tank and smooth operation cannot be conducted. This is a serious disadvantage.